The present invention relates to an exposure control device in an electrostatic image forming apparatus of a visible image transfer type designed to obtain exposure value for appropriate reproducibility when an electrostatic image is formed from an original image by such image forming apparatuses as copiers and laser beam printers.
The electrostatic image forming apparatus of a visible image transfer type forms an electrostatic latent image by projecting an original image on a photosensitive member, and then forms an image on a transfer sheet by transfer of the image after visualizing it through development. In the prior art, in order to obtain an appropriate image, toner supply, development bias, exposure value etc., are controlled singularly or in combination with each other. They are well known by the prior art described in the Japanese Pat. Publn. (KOKAI) No. 58-23043 (Conventional Device 1), the Japanese Pat. Publn. (KOKAI) No. 60-119589 (Conventional Device 2), the Japanese Pat. Publn. (KOKAI) No. 60-133475 (Conventional Device 3), the Japanese Pat. Publn. KOKAI No. 60-146256 (Conventional Device 4) and the Japanese Pat. Publn. (KOKAI) No. 60-260072 (Conventional Device 5). These apparatuses are divided into two types: ones which detect the density of a visible image formed on a photosensitive member and merely control the conditions for development or exposure to make the density of the visible image appropriate, or control such conditions based on the reference level corrected according to temperature, humidity, hysterises time for developing agent, variation in sensitivity of the photosensitive member (Conventional Devices 1, 3, 4 and 5); and, ones which control the exposure so as to reproduce an appropriate contrast in accordance with the contrast obtained from the detected density of an original image (Conventional Device 2). The apparatuses of the former type determine the density of the image to be reproduced irrespective of original density and contrast. Therefore, said method can not deal with certain problems; For example, fine lines which are thinly scattered in a pure white original are very often blurred when reproduced, and images crammed in a pure white original are damaged by fogging through reproduction. On the other hand, the apparatuses of the latter type have better reproducibility of fine lines, which is accomplished at low exposure lamp voltage. However, when dust is stuck to the optical system or the characteristics of the photosensitive member are changed, fogging on the background of the reproduced image for originals with white background, i.e. stains on the background, easily occurs because of the low lamp voltage. It is a significant feature of the system disclosed herein to overcome various of the disadvantages and limitations discussed above.